Wolf-Rayet star

photo: Hubble Space Telescope image of nebula M1-67 around Wolf-Rayet star WR 124

Wolf-Rayet stars (often referred to as WR stars) are evolved, massive stars (over 20 solar masses), which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s. While our own Sun loses approximately 10−14 solar masses every year, Wolf-Rayet stars typically lose 10−5 solar masses a year. Wolf-Rayet stars are very hot, with surface temperatures in the range of 25,000 K to 50,000 K. It is believed that the star in the galaxy NGC 2770 that exploded into a supernova on January 9, 2008 - the first supernova ever observed in the act of exploding - was a Wolf-Rayet star.

Observation history:

In 1867, astronomers using the 40 cm Foucault telescope at the Paris Observatory, discovered three stars in the constellation Cygnus (now designated HD191765, HD192103 and HD192641), that displayed broad emission bands on an otherwise continuous spectrum. The astronomer's names were Charles Wolf and Georges Rayet, and thus this category of stars became named Wolf-Rayet (WR) stars. Most stars display absorption bands in the spectrum, as a result of overlaying elements absorbing light energy at specific frequencies. The number of stars with emission lines is quite low, so these were clearly unusual objects.

The nature of the emission bands in the spectra of a Wolf-Rayet star remained a mystery for several decades. Edward C. Pickering theorized that the lines were caused by an unusual state of hydrogen, and it was found that this "Pickering series" of lines followed a pattern similar to the Balmer series, when half-integral quantum numbers were substituted. It was later shown that the lines resulted from the presence of helium; a gas that was discovered in 1868.

By 1929, the width of the emission bands was being attributed to the Doppler effect, and hence that the gas surrounding these stars must be moving with velocities of 300–2400 km/s along the line of sight. The conclusion was that a Wolf-Rayet star is continually ejecting gas into space, producing an expanding envelope of nebulous gas. The force ejecting the gas at the high velocities observed is radiation pressure.

In addition to helium, emission lines of carbon, oxygen and nitrogen were identified in the spectra of Wolf-Rayet stars.In 1938, the International Astronomical Union classified the spectra of Wolf-Rayet stars into types WN and WC, depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen respectively.

Description:

Wolf-Rayet stars are a normal stage in the evolution of very massive stars, in which strong, broad emission lines of helium and nitrogen ("WN" sequence) or helium, carbon, and oxygen ("WC" sequence) are visible. Due to their strong emission lines they can be identified in nearby galaxies. About 230 Wolf-Rayets are known in our own Milky Way Galaxy, about 100 are known in the Large Magellanic Cloud, while only 12 have been identified in the Small Magellanic Cloud.

Conti (1976) originally proposed that the WR stars as a class are descended from massive O-stars in which the strong stellar winds characteristic of extremely luminous stars have ejected the unprocessed outer H-rich layers. The characteristic emission lines are formed in the extended and dense high-velocity wind region enveloping the very hot stellar photosphere, which produces a flood of UV radiation that causes fluorescence in the line-forming wind region. This ejection process uncovers in succession, first the nitrogen-rich products of CNO cycle burning of hydrogen (WN stars), and later the carbon-rich layer due to He burning (WC & WO stars). Most of these stars are believed finally to progress to become supernovae of Type Ib or Type Ic. A few (roughly 10%) of the central stars of planetary nebulae are, despite their much lower (typically ~0.6 solar) masses, also observationally of the WR-type; i.e., they show emission line spectra with broad lines from helium, carbon and oxygen. Denoted [WR], they are much older objects descended from evolved low-mass stars and are closely related to white dwarfs, rather than to the very young, very massive stars that comprise the bulk of the WR class.

It is possible for a Wolf-Rayet star to progress to a "collapsar" stage in its death throes: This is when the core of the star collapses to form a black hole, pulling in the surrounding material. This is thought to be the precursor of a long gamma-ray burst.

The best known (and most visible) example of a Wolf-Rayet star is Gamma 2 Velorum (γ² Vel), which is a bright star visible to those located south of 40 degrees northern latitude. One of the members of the star system (Gamma Velorum is actually at least six stars) is a Wolf-Rayet star. Due to the exotic nature of its spectrum (bright emission lines in lieu of dark absorption lines) it is dubbed the "Spectral Gem of Southern Skies".

Fast jets shoot from a star

photo: Circinus X-1, the subject that influenced this illustration, is located about 20,000 light years from Earth in the constellation Circinus near the Southern Cross. NASA.

Date: January 29, 2004

A neutron star in a binary star system is spewing matter into space at nearly the speed of light.The recent discovery of a neutron star exhibiting behavior previously believed to be exclusive to black holes is challenging astronomers' understanding of the nature of some of the most extreme phenomena in the cosmos.

An international team of astronomers from the Netherlands, United Kingdom, and Australia used the Australia Telescope to study binary star system Circinus X-1 in radio waves during the past three years. Circinus X-1 consists of a star 3 to 5 times the mass of the Sun in a close orbital dance with a neutron star companion. The star system lies about 20,000 light-years away from Earth in our Milky Way Galaxy.

Since the 1970s, observers have noted Circinus X-1's emission of radio waves. Now, the team of astronomers, led by Rob Fender of the University of Amsterdam, has peered into the depths of the radio cloud and found something astonishing. Jets of matter are ejected out of the star system at 99.8 percent the speed of light.

These jets are the fastest ever observed shooting out of something other than a supermassive black hole in the center of a distant galaxy, and the fastest outflow ever observed originating within our own Milky Way.

The two stars circle each other once every 16.6 days, drawing tauntingly closer, then moving farther apart. As they dance, the neutron star — the end product of the violent explosion and collapse of a giant star — steals matter away from its stellar companion, forming an ever-growing accretion disk of hot gas in its outer atmosphere.

It seems that when the two stars get closest to one another, roughly every 17 days, hot matter from the accretion disk is spewed violently into interstellar space. The observation raises the question: What creates these enormously fast jets?

Astronomers previously attributed such speeding torrents to the environment characteristic of black holes, where space-time is warped beyond repair and gravity becomes infinitely potent. But if a neutron star can produce the same ultra-relativistic eruptions, then some more general set of circumstances must exist that accelerates matter to breakneck speeds.

"Whatever the physics underlying the production of ultra-relativistic jets — which are related to the jets of distant, massive galaxies and also probably gamma-ray bursts — it must be connected to the things that neutron stars and black holes have in common," says Fender.

He points to high densities, high pressures, and the build-up of magnetic fields as mutual characteristics and potential clues.

The observation is forcing astronomers to rethink their definitions of black holes and neutron stars, and to come to grips with the mechanism behind high-speed jets. It is at these most extreme conditions — where gravity contorts space and slows time — that fundamental physics comes to light.

Understanding the cause of these jets, then, is crucial.

"They are the fastest moving phenomena in the universe and probably are responsible for, or at least associated with, the biggest explosions since the Big Bang," Fender says.

That's why the team plans to continue studying Circinus X-1, to home in on the details.

"Since the outbursts go off every 16.6 days," explains Fender, "we can uniquely time the observations from ground and space-based facilities to observe at the moment of jet formation, and to study the physics more closely. This year, we will continue to make radio observations and hope to have long observations with the European Integral orbiting gamma-ray observatory."

Astronomers get best view yet of infant stars at feeding time

photo: Tracing gas emission close to young stellar objects (Artist view)

Friday, October 10, 2008

Astronomers have used ESO's Very Large Telescope Interferometer to conduct the first high resolution survey that combines spectroscopy and interferometry on intermediate-mass infant stars. They obtained a very precise view of the processes acting in the discs that feed stars as they form. These mechanisms include material infalling onto the star as well as gas being ejected, probably as a wind from the disc.

Infant stars form from a disc of gas and dust that surrounds the new star and, later, may also provide the material for a planetary system. Because the closest star-forming regions to us are about 500 light-years away, these discs appear very small on the sky, and their study requires special techniques to be able to probe the finer details.

This is best done with interferometry, a technique that combines the light of two or more telescopes so that the level of detail revealed corresponds to that which would be seen by a telescope with a diameter equal to the separation between the interferometer elements, typically 40 to 200 metres. ESO's Very Large Telescope Interferometer (VLTI) has allowed astronomers to reach a resolution of about a milli-arcsecond, an angle equivalent to the size of the full stop at the end of this sentence seen from a distance of about 50 kilometres.

"So far interferometry has mostly been used to probe the dust that closely surrounds young stars," says Eric Tatulli from Grenoble (France), who co-led this international project. "But dust is only one percent of the total mass of the discs. Their main component is gas, and its distribution may define the final architecture of planetary systems that are still forming."

The ability of the VLTI and the AMBER instrument to take spectra while probing objects at milli-arcsecond resolution has allowed astronomers to map the gas. Astronomers studied the inner gaseous environments of six young stars belonging to the family of Herbig Ae/Be objects. These objects have masses a few times that of our Sun and are still forming, increasing in mass by swallowing material from the surrounding disc.

The team used these observations to show that gas emission processes can be used to trace the physical processes acting close to the star.

"The origin of gas emissions from these young stars has been under debate until now, because in most earlier investigations of the gas component, the spatial resolution was not high enough to study the distribution of the gas close to the star," says co-leader Stefan Kraus from Bonn in Germany. "Astronomers had very different ideas about the physical processes that have been traced by the gas. By combining spectroscopy and interferometry, the VLTI has given us the opportunity to distinguish between the physical mechanisms responsible for the observed gas emission."

Astronomers have found evidence for matter falling into the star for two cases, and for mass outflow in four other stars, either in an extended stellar wind or in a disc wind.

It also seems that, for one of the stars, dust may be present closer to the star than had been generally expected. The dust is so close that the temperature should be high enough for it to evaporate, but since this is not observed, it must mean that gas shields the dust from the star's light.

These new observations demonstrate that it is now possible to study gas in the discs around young stars. This opens new perspectives for understanding this important phase in the life of a star.

"Future observations using VLTI spectro-interferometry will allow us to determine both the spatial distribution and motion of the gas, and might reveal whether the observed line emission is caused by a jet launched from the disc or by a stellar wind", concludes Stefan Kraus.

A Star That Bursts, Blinks and Disappears

photo: This illustration shows a flare from magnetar Swift J195509+261406. A starquake is probably what triggered the object's 40 optical flares.

Credit: NASA/Swift/Sonoma State University/A. Simonnet

"Twinkle, twinkle little star" goes the nursery rhyme. Now, astronomers are reporting on a strange case where one of the littlest of stars "twinkled" with gamma rays, X-rays, and light -- and then vanished.

The story began on June 10, 2007. That’s when a spike of gamma-rays lasting less than five seconds washed over NASA's Swift satellite. But this high-energy flash wasn't a gamma-ray burst -- the birth cry of a black hole far across the universe. It was something much closer to home.

Swift immediately reported the event’s position to astronomers all over the world. Within a minute, robotic telescopes turned to a spot in the constellation Vulpecula. Because Swift found an X-ray glow coming from this point, astronomers cataloged the object as "Swift J195509+261406," after its position in the sky and the discovering satellite. (Well, they had to call it something!)

During the next three days, the object brightened and faded in visible light. Not once, not twice -- but 40 times! Eleven days later, it flashed again, this time at infrared wavelengths. Then, it disappeared from view.

"I love it when Swift enables a discovery like this," says Neil Gehrels, the mission's lead scientist at NASA Goddard Space Flight Center in Greenbelt, Md. "The observatory is an astronomical robot built for gamma-ray burst studies, but it can also quickly point at other bizarre objects with bright flares."

Astronomers think the object was a neutron star -- the crushed innards of a massive star that long ago exploded as a supernova -- about 15,000 light-years away. Writing in the Sept. 25 issue of the science journal Nature, a team of 42 scientists concludes that Swift J195509+261406 is a special type of neutron star called a magnetar.

"We are dealing with an object that was hibernating for decades before entering a brief activity period," explains Alberto J. Castro-Tirado, lead author of the paper. "Magnetars remain quiet for decades."

Although measuring only about 12 miles across -- about the size of a city -- neutron stars have the strongest magnetic fields in the cosmos. Sometimes, those magnetic fields are super strong -- more than 100 times the strength of typical neutron stars.

Astronomers put these magnetic monsters in their own class: magnetars. Only about a dozen magnetars are known, but scientists suspect our galaxy contains many more. We just don’t see them because they’re quiet most of the time.

So what happened last year? Why did this previously unseen star begin behaving so badly? And why did it stop?

Combine a magnetar's pumped-up magnetic field with its rapid spin, and sooner or later something has to give. Every now and then, the magnetar’s rigid crust snaps under the strain.

This "starquake" releases pent-up magnetic energy, which creates bursts of light and radiation. Once the star’s crust and magnetic field settle down, the star goes dark and disappears from our view. At least until the next quake.

Astronomers suspect that magnetars lose their punch as time passes, but Swift J195509+261406 provides the missing link between objects exhibiting regular activity and those that have settled into retirement -- and invisibility.

So twinkle, twinkle magnetar. That's how we'll learn just where you are.

Astronomers Find Highly Elliptical Disk Around Young Star

This image taken by NASA's Hubble Space Telescope shows a lopsided debris disk around the young star HD 15115.

The disk, seen edge-on, is the dense blue line extending from the star to the upper right and lower left of the image. As seen from Earth, the edge-on disk resembles a needle sticking out from the star. The disk appears thicker and longer at upper right than at lower left, evidence of the disk's lopsided structure.

Astronomers think the disk's odd imbalanced look is caused by dust particles following a highly elliptical orbit around the star, which is slightly brighter than the Sun. The lopsidedness may have been caused by planets sweeping up debris in the disk or by the gravity of a nearby star.

Astronomers used an occulting mask on Hubble's Advanced Camera for Surveys to block out the bright starlight so they could see the dim disk. The occulting masks can be seen in the image as the dark circle in the center and the dark bar on the left. The star is behind the central mask.

HD 15115 is among nearly 30 stars that belong to the Beta Pictoris Moving Group. Moving groups are expanded clusters of stars believed to have a common birthplace and age, in this case about 12 million years, that are traveling together loosely through space. HD 15115 is 150 light-years from Earth.

Dusty disks are known to exist around at least 100 stars, but because of the difficulty in observing material close to the brightness of a star, less than a dozen have been studied closely.

Astronomers described the disk as one of the most peculiar debris disks that Hubble has ever imaged. They in fact made follow-up observations with the W.M. Keck Observatory in Hawaii to confirm the disk's presence.

Hubble's Advanced Camera for Surveys snapped the image on July 17, 2006.